Get rid of infinite recursion for 'typedef' used with GTY-marked 'gcc/diagnostic...

Get rid of infinite recursion for 'typedef' used with GTY-marked 'gcc/diagnostic-spec.h:nowarn_map' [PR101204, PR103157]

Reproduced with clang version 10.0.0-4ubuntu1:

    gtype-desc.c:11333:1: warning: all paths through this function will call itself [-Winfinite-recursion]

... as well as some GCC's '-O2 -fdump-tree-optimized':

    void gt_pch_nx(int_hash<unsigned int, 0u, 4294967295u>*, gt_pointer_operator, void*) ([...])
    {
      <bb 2>:

      <bb 3>:
      goto <bb 3>;

    }

That three-arguments 'gt_pch_nx' function as well as two one-argument
'gt_ggc_mx', 'gt_pch_nx' functions now turn empty:

    [...]
     void
    -gt_ggc_mx (int_hash<location_t,0,UINT_MAX>& x_r ATTRIBUTE_UNUSED)
    +gt_ggc_mx (struct xint_hash_t& x_r ATTRIBUTE_UNUSED)
     {
    -  int_hash<location_t,0,UINT_MAX> * ATTRIBUTE_UNUSED x = &x_r;
    -  gt_ggc_mx (&((*x)));
    +  struct xint_hash_t * ATTRIBUTE_UNUSED x = &x_r;
     }
    [...]
     void
    -gt_pch_nx (int_hash<location_t,0,UINT_MAX>& x_r ATTRIBUTE_UNUSED)
    +gt_pch_nx (struct xint_hash_t& x_r ATTRIBUTE_UNUSED)
     {
    -  int_hash<location_t,0,UINT_MAX> * ATTRIBUTE_UNUSED x = &x_r;
    -  gt_pch_nx (&((*x)));
    +  struct xint_hash_t * ATTRIBUTE_UNUSED x = &x_r;
     }
    [...]
     void
    -gt_pch_nx (int_hash<location_t,0,UINT_MAX>* x ATTRIBUTE_UNUSED,
    +gt_pch_nx (struct xint_hash_t* x ATTRIBUTE_UNUSED,
            ATTRIBUTE_UNUSED gt_pointer_operator op,
            ATTRIBUTE_UNUSED void *cookie)
     {
    -    gt_pch_nx (&((*x)), op, cookie);
     }
    [...]

	gcc/
	PR middle-end/101204
	PR other/103157
	* diagnostic-spec.h (typedef xint_hash_t): Turn into...
	(struct xint_hash_t): ... this.
	* doc/gty.texi: Update.